Measurement of nonlinear observables in the Large Hadron Collider using kicked beams

Abstract

The nonlinear dynamics of a circular accelerator such as the Large Hadron Collider (LHC) may significantly impact its performance. As the LHC progresses to more challenging regimes of operation it is to be expected that the nonlinear single particle dynamics in the transverse planes will play an increasing role in limiting the reach of the accelerator. As such it is vital that the nonlinear sources are well understood. The nonlinear fields of a circular accelerator may be probed through measurement of the amplitude detuning: the variation of tune with single particle emittance. This quantity may be assessed experimentally by exciting the beam to large amplitudes with kicks, and obtaining the tunes and actions from turn-by-turn data at Beam Position Monitors. The large amplitude excitations inherent to such a measurement also facilitate measurement of the dynamic aperture from an analysis of beam losses following the kicks. In 2012 these measurements were performed on the LHC Beam 2 at injection energy (450 GeV) with the nominal magnetic configuration. Nonlinear coupling was also observed. A second set of measurements were performed following the application of corrections for b 4 and b 5 errors. Analysis of the experimental results, and a comparison to simulation are presented herein.

Collider Beam Physics

“…The actual LHC dynamic aperture at injection was measured with the help of kicked beams [98]. The measured and simulated dynamic aperture values agree well.…”

“…The measured and simulated dynamic aperture values agree well. It was also shown that through a minimization of readily observed nonlinear parameters it is possible to increase the LHC dynamic aperture [98].…”

Amplitude dependent closest tune approach

“…Compelling experimental observations in the LHC [5] suggested the existence of an amplitude dependent closest tune approach. These experimental observations have been further complemented with computer simulations of the LHC in [6] identifying the key ingredients to reproduce the observations, namely linear coupling and normal octupole fields.…”

First measurement and correction of nonlinear errors in the experimental insertions of the CERN Large Hadron Collider

“…However, upon reduction of the LHC β Ã from 1 to 0.6 m in 2012, a significant degradation of the beam lifetime was observed [12]. Optics studies at injection [13] and top energy [14] also revealed larger than expected discrepancies between the measured and predicted first and second order amplitude detuning. Nonlinear errors in the triplets and separation dipoles are one of several candidates for the source of these reductions in performance.…”